1. Field of the Invention
The present invention relates to a cover for an air bag installation.
2. Description of the Related Art
It is now common to provide, within a vehicle such as a motor car, air bags located in front of the driver and passenger, the air bags being adapted to inflate in the event that an accident arises.
Typically an air bag is retained within an installation which defines a cavity containing the folded air bag with the cavity having a cover.
Various designs of covers have been proposed previously, including a cover which comprises an inner relatively stiff base layer moulded from a plastic material and an outer softer layer, defining the outer surface of the cover, being formed of a different polymer. In such a case, it is preferred that the base layer is initially moulded, and the base layer is then introduced into a further mould where the soft polymer is moulded into position on the base. It is preferred to utilise thermoplastic polymers for both the base layer and the outer layer. An arrangement of this type is described in DE-A-394264.
It is to be appreciated that for aesthetic reasons it may be desired to give the outer surface of the cover a certain shape, and for practical reasons the inner surface of the base layer must have a specific configuration, and as a consequence the total thickness of the cover may well have to vary from point to point in the cover.
Where the thickness of the cover varies it is an obvious design expedient to provide at least one of the two layers with a variable thickness. However, if the thickness of the material in either of the layers becomes relatively great, a problem may well arise with shrinkage on solidification, since typical shrinkage is about 1.5%. Also, of course, with increasing thickness of either layer solidification at the end of the moulding process will take a longer time, leading to a slower moulding rate.